1. Field of the Invention
This invention relates to the field of solid state electronics and particularly to the field of imaging charge coupled devices (CCDs).
2. Description of the Prior Art
Charge coupled devices (CCDs) previously used for visible imagers have typically consisted of an n-MOS Si CCD channel on a p-Si absorber. While this structure is adequate for many applications, in some specific applications the performance of Si CCD imagers is hampered by the fundamental material properties of Si and of SiO.sub.2.
One such application is for star sensors which require extremely low dark currents and high optical responsivity at elevated temperatures in an irradiated environment. These goals are not likely to be realized simultaneously in a Si CCD. To achieve low dark currents in a Si CCD, long lifetime and thus long diffusion length material must be used. The diffusion length in Si is much longer than a pixel width, resulting in cross-talk between the pixels. To minimize the cross-talk, the substrate (absorber) is thinned forcing a trade-off between optical response and spatial resolution. While the 1.1 eV bandgap of Si provides a fair match to the visible spectrum, it is also the source of relatively high dark currents. Additionally, the use of an oxide and a deep active layer make Si CCDs extremely sensitive to the effects of radiation.
By making use of the well developed heteroepitaxial technology, compound semiconductors such as (GaAl)As can be used to fabricate structures in which optical absorption and charge transfer are performed in adjacent epilayers of different materials. The material properties of each region can be individually adjusted to optimize overall device performance. One of the problems encountered in applying this heteroepitaxial technology to imaging CCDs is the presence of the inactive semiconductor substrate. The substrate serves as a starting material for growth of the thin epitaxial layers and it provides a support for them. However, it has no functional role in the device operation. Because it is absorbing to the spectral region of interest, the device must be illuminated from the opposite side through the CCD channel layer. Such "frontside" illumination using transparent Schottky gates is rather complicated and has low quantum efficiency.